bug in conditional forecast

The conditional forecast is buggy.

Namely there is some inconsistency in indexing between decision rule and declaration order.

The way to fix it is to make sure that the pre-processor sets the vector of indices: constrained_vars_ in decision rule order. In fact, imcforecast assumes that constrained_vars_ is in decision rule order Moreover, one needs to fix imcforecast for the trend, since the latter is already defined in decision-rule ordering (so no need to use inv_order_var). I am going to make a pull request with the imcforecast fix, but I do not touch the pre-processor. If someone (@houtan?) could help with the latter. Many thanks